Compression-ignition engine operation



United States Homer M. Fox, Bartlesville, Okla., assignor to Phillips Petroleum Company, a corporation of Delaware No Drawing. Application December 20, 1957 Serial No. 704,014

19 Claims. (Cl. 123-1) This invention relates to compression-ignition engine operation. In one aspect this invention relates to the op eration of a diesel engine with a fuel having improved ignition characteristics. In another aspect this invention relates to the operation of a diesel engine with a fuel containing a minor proportion of an organic sulfenamide. In another aspect this invention relates to a compression ignition engine fuel containing a small but effective proportion of an organic sulfenamide, defined hereinafter.

This application is a continuation-in-part of my copending application Serial No. 397,771, filed December 11, 1953, now Patent 2,877,749.

For compression-ignition engines, of which the wellknown diesel engine is representative, it is desirable to have a fuel which has a high cetane number in order to prevent knocking, smoking, and other harmful results which usually attend the use of low cetane number fuels.

in engines operating on the diesel cycle, where the fuel charge is injected into compressed air in a combustion space, it is important to the attainment of maximum efliciency that the delay period between injection and ignition be short. Enhanced speed of ignition in a diesel fuel is, therefore, a highly desirable characteristic. Improperly delayed ignition in a diesel engine gives rise to the phenomena known as knocking, due to improper coordination of combustion and cylinder conditions. By properly adjusting the ignition delay period of the fuel, through acceleration of the ignition, it is possible to lower the compression pressure which will insure spontaneous ignition without combustion shock, thereby increasing the ease of starting and the smoothness with which combustion takes place. The low compression pressure permits a lower weight in the engine and a lower cost per unit power output.

It is a Well-known fact that many petroleum oil fractions have a suitable boiling range and other characteristics which, in general, would make them suitable as diesel fuels, but which unfortunately are not sufficiently high in cetane number to give satisfactory performance. This is particularly true in regard to diesel engines operating at very high speeds such as over one thousand r.p.m. The primary effect of low cetane number is increased ignition lag. This, in turn, can lead to knock, low combustion efficiency, and difiicult low temperature starting. It has, therefore, been proposed in the past to incorporate in the petroleum oil fraction to be used as diesel fuel, a small amount of an addition agent, usually called an ignition promoter or diesel dope, which is adapted to cause a substantial increase in the cetane number of the fuel base stock. A large number of such ignition promoters have been proposed in the past, but many such promoters possess inherent disadvantages which more than offset the value in raising the cetane number. For instance, ethyl nitrate is very effective for improving the cetane number but is such a volatile liquid that, when added in substantial quantities to a diesel fuel base stock, it causes an undesirable lowering of the flash point of the fuel. On the atent ice other hand, some other types of ignition promoters which are very effective in raising the cetane number of the fuel have very harmful characteristics such as causing corrosion of the fuel tank.

The following objects of this invention will be attained by the various aspects of this invention.

An object of the invention is to provide improved compression-ignition engine operation. Another object of the invention is to provide improved diesel engine operation with a fuel having improved ignition characteristics. Another object of the invention is to provide an improved diesel engine fuel. Another object of the invention is to provide improved diesel engine operation with a fuel combustion in which the ignition delay characteristic is shortened by the incorporation therein of an organic sulfenamide. Other aspects, objects and advantages of this invention will be apparent to those skilled in the art upon study of the accompanying disclosure.

The invention in said copending application resides broadly in the discovery that the cetane value of a compression ignition engine fuel is improved by the incorporation therein of a minor amount of an organic sulfenamide. The efliciency of operation of a diesel engine is materially improved when a fuel containing a minor amount of an organic sulfenamide is burned in a combustion chamber of the engine. The sulfenamide, either individual organic sulfenamides or mixtures thereof, is incorporated in the diesel fuel, at some time prior to in jection into the combustion zone, in an amount within the range of between 0.1 and 10 volume percent, generally within the range of between 0.5 and 5 volume percent.

The ignition quality or the case with which a diesel fuel is ignited after it is injected into the cylinders of a diesel engine is measured in terms of cetane number. Cetane number is defined as the percentage of n-cetane in a mixture of alpha-methylnaphthalene and n-cetane which has an ignition quality comparable to the fuel in question. The use of fuels of poor quality results in starting difficulty and detonation, knocking, or rough running. Many additives have been developed for blending with diesel fuel in a comparatively small amount to impart a high cetane value to the fuel. Compounds which have been proposed as additives for such fuels include peroxides, nitrates of glycols, and lower molecular weight alkyl nitrates and nitrites.

The fuel base stock to be used is preferably a gas oil boiling from about 350 F. to 750 F. In a broader aspect, the fuel is one which has a suitable boiling range and viscosity for use as a fuel in a diesel type engine. Under some circumstances, a more normally cut fraction, such as one distilling from 400 F. to 600 F. or from 500 F. to 700 F., may be used.

Although oil of the parafiinic type or hydrogenated oils are preferred for fueling compression-ignition engines, such fuels are generally more expensive and are not always available or practical. Diesel fuels giving satisfactory engine performance are formed from crude oils, gas oil, and residual fuels having low pour point, low A.P.I. gravity and high heat value, such as are procured from naphthenic base, asphaltic base, or mixed base stock. Heavier and lower grades of petroleum oil, such as residual oils, are made suitable for a slow-speed engine. On the other hand, for high-speed engines, such as those operating at speeds above 1,000 r.p.m., it is probably best to use a fuel base stock rich in parafiinic hydrocarbons or hydrogenated oil and still further improve the cetane number thereof.

I have discovered that organic sulfenamides, when added to a diesel fuel in relatively minor amounts, have the effect of materially increasing the cetane value of such a fuel. The organic sulfenamides are made up of atoms selected from the group consisting of carbon, hydrogen, sulfur, nitrogen and oxygen. More specifically, I have discovered that organicsulfenamides, particularly sulfenamides having more than five and not more than 24, preferably not more than 17, carbon atoms per molecule, are suitable for use as the ignition improver of the invention disclosed and claimed in said copending, application.

One group of said sulfenamides disclosed and claimed in said copending application comprises compounds which are characterized by the structural formula wherein X is selected from the group consisting of a hydrogen atom and an SR group, wherein R is an alkyl or cycloalkyl group containing from 1 to 8 carbon atoms and wherein R is an alkyl group containing from 1 to 7 carbon atoms. Typical compounds within this group include N-normal butyl-S-tertiary-butyl sulfenamide, N- cyclohexyl-S-tert-butyl sulfenamide and the like.

Another group of said organic sulfenamides which are suitable for use as ignition improvers and which are disclosed and claimed in said copending application are characterized by the structural formula H H; C-C R-(S).,.-N/ \X H2 Hz wherein R is an alkyl group containing from 2 to 12 carbon atoms and wherein X is a methylene group, a nitrogen-atom, a sulfur atom or an oxygen atom and wherein n is a whole integer selected from a group of 1 or 2 and wherein when X is an oxygen atom, n is 2. Typical compounds Within this group include tert-butyl thiosulfenylpiperidine, isopropylsulfenylpiperidine, tert-butyl thiosulfenylmorpholine, and the like.

Still another group of organic sulfenamides which are disclosed and claimed in said copending application as suitable for use as ignition improvers. include those sulfenamides characterized by the formula wherein X is a hydrogen atom or an SR group, at least one of which is an SR group, wherein X is a hydrogen atom or an alkyl group containing not more than 6 carbon atoms, wherein R is a hydrogen atom or a methyl radical and n is a numeral selected from the group of through 5, inclusive, and wherein R is an alkyl group containing from 2 to 12 carbon atoms. Typical compounds within this group include ethylene di(tertiarybuty-l sulfenamide), propylene di(tertiary-butyl sulfenamide), triethylene tetra (tertiary-butyl sulfenamide), tetraethylene penta (tertiary-butyl sulfenamide), and the like.

I have now discovered another group of organic sulfenamides which are suitable for use as ignition improvers. Thus the present invention comprises a compression ignition engine fuel, and a method of operating a compression ignition engine using said fuel, which fuel comprises a liquid hydrocarbon fuel containing from 0.1 to percent by volume of an organic sulfenamide selected from the group of sulfenamides characterized by the formula wherein each R is selected from the group consisting of alkyl and cycloalkyl groups containing from 1 to 8 carbon atoms, and R is an alkyl group containing from 1 to 7 carbon atoms. Said sulfenamides, particularly those containing more than five but not more than 23 preferably not more than 17, carbon atoms per molecule are suitable for use as the ignition improver of the instant invention. Presently preferred sulfenamidcs are those wherein R and R' are lower alkyl groups containing from 1 to 4 carbon atoms.

Typical sulfenamides of the present invention include, among others, the following: N,N-dimethyl isopropylsulfenamide; N,N-dimethyl n-propylsulfenamide; N-ethyl-N- methyl ethylsulfenamide; N,N-diethyl methylsulfenamide; N,N-dimethyl tert-butyl-sulfenamide; N-ethyl-N-methyl tert-butyl-sulfenamide; N,N-dimethyl sec-butyl-sulfenamide; N,N-dimethyl n-butylsulfenamide; N,N-bis(te1tbutyl) isopropylsulfenamide; N,N-diethyl tert-butylsulfenamide; N,N-diisopropyl tert-butylsulfenamide; N,N-diisopropyl cyclopentylsulfenamide; N,N-dicyclopentyl n-hexylsulfenamide; N,N-di-tert butyl n pentylsulfenarnide; N,N-bis-(Z-hexyl) n-pentylsulfenamide; N-(2-pentyl)-N- (3-heptyl) 2-pentylsulfenamide; N,N-dicyclohexyl nheptyls-ulfenamide; N,N-bis(2-heptyl) 3.-hexylsulfenamide; and N,N-bis(3-octyl) n-heptylsnlfenamide.

The sulfenamides used in the fuel of the present invention are known'in the art. U.S. Patent 2,520,400 to C. M. Hirnel et a1. describes the production of sulfenamides by the interaction of sulfenyl halides with amines in the presence of an alkaline material, such as sodium hydroxide, or the like. U.S. Patent 2,520,401 to C. M. Himel et a1. describes the production of thiosulfenamides by the interaction of thiosulfenyl halides with amines in the presence of an alkaline material, such as sodium hydroxide. A further discussion of the methods of preparing sulfenamides may be found in Chemical Reviews 39, 304-323 (1946).

It is, of course, understood that other known addition agents such as oiliness agents, sludge dispersers, colloidal suspension dispersers, stabilizers, dyes, anti-oxidants, vis cosity improvers, pour point depressors, solubilizers, gum solvents, and corrosion inhibitors may be used in the fuels of the invention.

This invention is exemplified by the following example which is not intended to unduly limit the invention Example The effectiveness of the sulfenamides of the invention in increasing the cetane number of diesel fuel is illustrated by comparing the cetane numbers of two base diesel fuels before and after adding 0.5 volume percent of N,N-dimethyl tert-butyl sulfenamide to each. The cetane numbers were determined by the procedure of the ASTM Committee D-2 on Petroleum Products and Lubricants entitled, Tentative Method of Tests for Ignition Quality of Diesel Fuels by the Cetane Method, and designated as Method D6l3-48T. The base fuels used were commercial diesel fuels having the properties set forth below. The improvement of cetane number of each base fuel is shown in the table below.

The two base diesel fuelsused in the above tests had the following properties.

Various modifications of this invention will be apparent to those skilled in the art upon study of this disclosure and are believed to be within the spirit and the scope of the invention.

I claim:

1. A compression-ignition engine fuel which comprises a liquid hydrocarbon fuel containing from 0.1 to volume percent of an organic sulfenamide characterized by the formula wherein each R is selected from the group consisting of alkyl and cycloalkyl groups containing from 1 to 8 carbon atoms, and R is an alkyl group containing from 1 to 7 carbon atoms.

2. The fuel of claim 1 wherein said liquid hydrocarbon is a gas oil boiling within the range of about 350 to about 750 F.

3. The fuel of claim 1 wherein N,N-dimethyl tert-butylsulfenamide.

4. The fuel of claim 1 wherein N,N-dimethyl isopropylsulfenamide.

5. The fuel of claim 1 wherein said N-ethyl-N-methyl tert-butylsulfenamide.

6. The fuel of claim 1 wherein said N,N-diethyl tert-butylsulfenamide.

7. The fuel of claim 1 wherein said N,N-diisopropyl tert-butylsulfenamide.

8. The fuel of claim 1 wherein said N,N-dimethyl n-propylsulfenamide.

said sulfenamide is said sulfenamide is sulfenamide is sulfenarnide is sulfenamide is sulfenamide is 9. The fuel of claim 1 wherein said sulfenamide is N,N-dimethyl sec-butylsulfenamide.

10. The fuel of claim 1 wherein said sulfenamide is N,N-bis tert-butyl) isopropylsulfenamide.

11. The method of operating a compression-ignition engine, which method comprises: compressing a charge comprising air in a cylinder of said engine; and injecting a liquid hydrocarbon fuel having an organic sulfenamide in an amount within the range of 0.1 to 10 volume percent admixed therewith into said compressed charge, said amount being sufiicient to accelerate the ignition of said fuel, and said sulfeziamide being characterized by the formula wherein each R is selected from the group consisting of alkyl and cycloalkyl groups containing from 1 to 8 carbon atoms, and R is an alkyl group containing from 1 to 7 carbon atoms.

12. The method of claim 11 wherein said sulfenamide is N,N-dimethyl tert-butylsulfenamide.

13. The method of claim 11 wherein said sulfenamide is N,N-dimethyl isopropylsulfenamide.

14. The method of claim 11 wherein said sulfenamide is N-ethyl-N-methyl tert-butylsulfenamide.

15. The method of claim 11 wherein said sulfenamide is N,N-diethyl tert-butylsulfenamide.

16. The method of claim 11 wherein said sulfenamide is N,N-diisopropyl tert-butylsulfenamide.

17. The method of claim 11 wherein said sulfenamide is N,N-dimethyl n-propylsulfenamide.

18. The method of claim 11 wherein said sulfenamide is N,N-dimethyl sec-butylsulfenamide.

19. The method of claim 11 wherein said sulfenamide is N,N-bis(tert-butyl)isopropylsulfenamide.

References Cited in the file of this patent UNITED STATES PATENTS 2,169,186 Shoemaker et al Aug. 8, 1939 2,177,719 James et al. Oct. 31, 1939 2,218,447 Baderscher et al. Oct. 15, 1940 2,421,352 Paul et a1. May 27, 1947 2,685,502 Heisig Aug. 3, 1954- 

11. THE METHOD OF OPERATING A COMPRESSION-IGNITION ENGINE, WHICH METHOD COMPRISES: COMPRESSING A CHARGE COMPRISING AIR IN A CYLINDER OF SAID ENGINE; AND INJECTING A LIQUID HYDROCARBON FUEL HAVING AN ORGANIC SULFENAMIDE IN AN AMOUNT WITHIN THE RANGE OF 0.1 TO 10 VOLUME PERCENT ADMIXED THEREWITH INTO SAID COMPRESSED CHARGE, SAID AMOUNT BEING SUFFICIENT TO ACCELERATE THE IGNITION OF SAID FUEL, AND SAID SULFEMAMIDE BEING CHARACTERIZED BY THE FORMULA 